The present invention relates to lasers, and, more particularly, to a system and method for determining the wavefront slope of a laser beam.
Wavefront slope sensors are used, for example, in adaptive optical systems to determine the adjustment necessary to compensate for wavefront distortions produced by the optical train and/or the intervening medium. In most applications, the ideal is a zero wavefront slope over the cross-section of the beam. A non-zero slope indicates distortion and dispersion of the beam, the result being a diminished power delivery.
The outgoing beam can be predistorted, for example, by varying the pressure applied across a deformable primary mirror, as a function of the measured slope distribution. In this way, an appropriate slope-related parameter can be minimized to enhance the power delivered.
The criteria according to which wavefront slope sensors are evaluated include reliability, speed, resolution and cost. Another primary consideration is the flexibility for use with both pulsed and continuous wave lasers.
Several of the available or described slope sensors include moving elements. The movements may be used to sample different parts of the beam, or to provide multiple measurements of a single part of the beam as necessary to compute the slope. Such systems are disadvantageous in that they are generally limited to continuous wave applications. Also, the time involved in the mechanical movements limits the measurement speed of the system. This is particularly problematic where real-time adjustments are required. Furthermore, the incorporation of moving parts reduces the reliability and increases the cost of the system.
There is another class of wavefront slope sensors which use multiple subapertures in sampling the laser beam. For example, Hartman-type sensors use a grating which defines multiple subapertures The average slope within each subaperture is determined, and the collective determination provide a slope distribution for the beam.
The major problem with the grating sensors is the difficulty of manufacturing high resolution gratings. Additionally, a significant percentage of the incident light can be lost by reflections at the grating. Also, diffraction effects become more severe at smaller apertures. Finally, it turns out that the mechanical aperture sensors have greater computational requirements by about a factor of three or four. Thus, real-time performance is impaired.
Heretofore, the disclosed and available sensors have provided a choice between the limitations of systems with mechanical movement and those with mechanical grids of subapertures. What is needed is reliable, cost-effective, fast wavefront sensing which efficiently processes data, provides for high-resolution sensing, and is applicable to pulsed as well as continuous wave applications.